Diesel (compression ignition) engines are among the most energy-efficient engines available, with admirably high power output per fuel consumption. Unfortunately, they're also among the “dirtiest” engines available, with common diesel engines (at the time of this document's preparation) being prone to high production of nitrogen oxides (commonly denoted NOx), which result in adverse effects such as smog and acid rain, and unburned hydrocarbon particulates (often simply called “soot”), sometimes seen as the black smoke emitted by a diesel vehicle as it accelerates from a stop. Soot particularly tends to be a problem when diesel engines are idling or operating at low loads, and for this reason, many areas of the United States have adopted “anti-idling” laws limiting the amount of time that a diesel vehicle can be left idling.
Because of the impact of soot and NOx emissions on the environment, the United States and many other countries have also imposed stringent emissions regulations on the use of diesel engines in vehicles, and numerous technologies have been developed which attempt to reduce diesel emissions. As an example, NOx is generally associated with high-temperature engine conditions, and may therefore be reduced in the engine by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (typically after cooling the exhaust gas), thereby reducing the oxygen in the combustion chamber and reducing the maximum combustion temperature. As another example, soot is generally associated with incomplete combustion, and can therefore be reduced in the engine by increasing combustion temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NOx production in an engine tend to increase soot production, and measures which reduce soot production in an engine tend to increase NOx production, resulting in what is often termed the “soot-NOx tradeoff.”
NOx and soot can also be addressed after they leave the engine (e.g., in the exhaust stream), but such “after-treatment” methods tend to be expensive to install and maintain. As examples, the exhaust stream may be treated with catalysts and/or injections of urea or other reducing/reacting agents to reduce NOx emissions, and/or fuel can periodically be injected and ignited in the exhaust stream to burn off soot collected in “particulate traps.” These approaches require considerable expense and complexity, and in the case of particulate traps, they tend to reduce a vehicle's fuel efficiency.
Other technologies have more fundamentally focused on how to reduce both NOx and soot generation from the combustion process and thereby obtain cleaner “engine out” emissions (i.e., emissions directly exiting the engine, prior to exhaust after-treatment or similar measures). These approaches include modifying the timing, rate, and/or shape of fuel injection charges, modifying the combustion chamber shape, and/or modifying other factors to try to attain complete combustion of all fuel (and thus lower soot) while controlling the combustion temperature (thus controlling NOx). One family of approaches can be broadly referred to as Low-Temperature Combustion strategies, which seek to avoid the rapid ignition conditions (rapid temperature and pressure rises) favorable to NOx and soot production. LTC strategies include Homogeneous Charge Compression Ignition (HCCI), which attempts to obtain a fully premixed (i.e., homogeneous) fuel-air mixture within the cylinder, typically via introduction of fuel charges into the cylinder very early in the combustion cycle; and Premixed Charge Compression Ignition (PCCI) and Partially Premixed Combustion (PPC), variants of HCCI wherein at least some of the fuel is injected into the cylinder to stratify the fuel distribution, and thus the equivalence ratio, across the cylinder. (It should be understood when reviewing the literature that nomenclature is not well-settled, with the aforementioned HCCI methods sometimes being referred to as “PCCI,” and PCCI methods sometimes being referred to as “HCCI,” or with the same methods being given different names entirely.) Many of these technologies provide emissions improvements, but are difficult to implement and control, particularly over the complete range of speeds and loads over which common diesel vehicle engines must operate. Additionally, many of these technologies still require measures such as exhaust after-treatment to attain emissions targets, leading to the aforementioned issues with cost and fuel efficiency.
Because of the difficulties in complying with emissions regulations while providing the fuel efficiency, cost, and performance that consumers seek, many automotive companies have simply shifted their focus away from diesel engines to the use of gasoline engines. Gasoline engines unfortunately have lower energy efficiency, and their emissions are also of concern. (For the reader having limited familiarity with internal combustion engines, the primary difference between gasoline engines and diesel engines is the manner in which combustion is initiated. Gasoline engines—also commonly referred to as spark ignition or “SI” engines—provide a relatively fuel-rich mixture of air and fuel into an engine cylinder, with a spark then igniting the mixture to drive the piston outwardly from the cylinder to generate work. In diesel engines—also known as compression ignition engines—fuel is introduced into an engine cylinder as the piston compresses the air therein, with the fuel then igniting under the compressed high pressure/high temperature conditions to drive the piston outwardly from the cylinder to generate work.)
Prior U.S. patents such as U.S. Pat. No. 8,616,177 to Reitz et al. (and related U.S. Pat. No. 9,080,501 to Reitz et al.), U.S. Pat. No. 8,851,045 to Reitz et al., and U.S. Pat. No. 9,057,321 to Reitz et al. describe diesel combustion methods, now referred to as Reactivity-Controlled Compression Ignition (RCCI) methods, wherein the fuel provided to the engine's combustion chamber is adapted to have its reactivity vary over the course of a combustion cycle. (“Reactivity” is a property corresponding to a fuel's tendency to spontaneously ignite under diesel operation conditions, i.e., under high pressures and temperatures. Thus, reactivity generally corresponds to a fuel's cetane number, or the converse of the fuel's octane number). In these RCCI methods, the fuel is also provided to the combustion chamber in such a manner that a stratified distribution of fuel reactivity results, that is, spaced regions of high reactivity and low reactivity are situated within the combustion chamber during the compression stroke of the combustion cycle. During compression, the higher-reactivity regions ignite first, with combustion then propagating to the lower-reactivity regions. With appropriate tailoring of fuel reactivity, fuel/reactivity amounts and proportions, the timing of fuel introduction into the combustion chamber, and similar factors, combustion can be tailored to produce peak work output at the desired time (for optimal power output), with low NOx and soot production. Experimental engines implementing RCCI methods resulted in exceptionally high fuel efficiency while meeting U.S. government emissions standards applicable at that time, without the need for exhaust gas after-treatment.
An exemplary version of the above-noted RCCI methods is shown in greater detail in FIGS. 1A-1D. An initial fuel charge having a first reactivity is supplied to the combustion chamber during the intake and/or compression stroke sufficiently prior to Top Dead Center (TDC) that the initial fuel charge is at least partially premixed (homogeneously dispersed) within the combustion chamber before the following injection(s) is/are made. The initial charge may be introduced into the combustion chamber via (preferably low-pressure) direct injection into the cylinder, and/or by providing it through the combustion chamber's intake port, as by injecting or otherwise introducing the charge into the intake manifold, and/or into an intake runner extending therefrom. The accompanying FIG. 1A schematically illustrates the premixed initial fuel charge within the combustion chamber during the compression stroke, with the piston traveling toward TDC.
One or more following fuel charges of different reactivity can thereafter be supplied to the combustion chamber in such a manner that a stratified distribution of fuel reactivity results within the combustion chamber, with distinct regions of higher and lower fuel reactivity. More specifically, the later different-reactivity charges are timed and otherwise designed to distribute the different-reactivity charges—which will be introduced into the premixed “matrix” of air and first-reactivity fuel—in such a manner that the reactivity gradient within the combustion chamber provides desired combustion phasing (combustion start time, rate, and duration resulting in controlled heat release and superior work input to the piston), while deterring rapid pressure increases and high temperatures (which promote NOx production and reduce fuel economy), and while completely burning all (or nearly all) fuel within the combustion chamber to reduce soot. Typically, a following high-reactivity fuel charge is supplied to the combustion chamber between the time the intake port is closed and approximately 40 degrees before TDC. For a typical combustion chamber which is partially bounded by a piston face with a central bowl (as depicted in FIGS. 1A-1D), the following fuel charge is preferably introduced at such a time (and with such pressure) that at least a major portion of the following fuel charge is directed toward an outer (squish) region located at or near an outer radius of the piston face. More specifically, the fuel charge is directed toward a region located outside of an outer third of the radius of the piston face. This is exemplified by FIG. 1B, which shows the combustion chamber at approximately 60 degrees before TDC, and with an injection being directed by the injector toward the squish region. However, in all instances injection is always preferably provided at pressures which avoid or minimize charge impingement on combustion chamber surfaces, since such impingement tends to enhance soot production.
FIG. 1C then depicts the injection of a second following high-reactivity fuel charge into the combustion chamber at approximately 30 degrees before TDC, with at least a major portion of the injection being directed toward an inner (bowl) region spaced inwardly from the outer radius of the piston face. More specifically, at least a major portion of the fuel charge is preferably injected toward a region located inside an outer fourth of the radius of the piston face (i.e., it is injected toward a region defined by the inner 75% of the bore radius). In the meantime, the first following fuel charge has begun to diffuse from the squish region, and to mix with the low-reactivity fuel from the initial fuel charge to form a region of intermediate reactivity at or near the squish region.
FIG. 1D then illustrates the combustion chamber of FIG. 1B at approximately 15 degrees before TDC, with the fuel in the chamber having a reactivity gradient ranging from higher-reactivity regions in the bowl to lower-reactivity regions at the outer diameter of the chamber, and at the crown of the bowl. Combustion may begin around this time, starting at the regions of highest reactivity (these regions being generated via the introduction of the higher-reactivity material), and spreading therefrom to the lower-reactivity regions via volumetric energy release and/or flame propagation until all fuel from all charges is consumed. Greater stratification/gradation in reactivity tends to result in a lower combustion rate. Conversely, lower stratification/gradation in reactivity (greater uniformity in reactivity throughout the combustion chamber) tends to result in a higher combustion rate, since each location within the chamber has an approximately equal chance of igniting first, and those locations that do not ignite first will be rapidly ignited by their neighboring locations.
Basically the same combustion mechanism results if the reactivities of the charges of FIGS. 1A-1D are reversed, i.e., if one or more initial higher-reactivity charges are followed by one or more following lower-reactivity charges: ignition begins in the higher-reactivity regions and propagates to the lower-reactivity regions. Combustion phasing can be controlled by the timings and amounts of the fuel charges, which affect the degree of stratification attained. For optimal work output, it is desirable that the fuel charges are supplied to the combustion chamber to attain peak cylinder pressure at or after Top Dead Center (TDC), more preferably between TDC and 20 degrees ATDC (After TDC), and most preferably between 5 and 15 degrees ATDC. In similar respects, CA50 (i.e., 50% of the total fuel mass burned) preferably occurs between approximately 0 to 10 degrees ATDC. It is also useful to supply the fuel charges in such a manner that the rate of pressure rise is no greater than 10 bar per degree of crank angle rotation, since greater pressure rise can generate unwanted noise and more rapid engine wear, and also promotes higher temperatures (and thus increased fuel consumption owing to heat transfer losses, as well as NOx production).
The different fuel charges, with their differing reactivities, can be conventional fuels supplied to the engine from separate conventional tanks, e.g., diesel fuel (which has a higher reactivity) from one tank, and gasoline (which has lower reactivity) from another tank. Alternatively or additionally, fuel from a single tank can have its reactivity modified between higher and lower levels by the addition of an appropriate reactivity modifier. As an example, an initial lower-reactivity charge could simply contain gasoline or diesel fuel, and a following higher-reactivity fuel charge could contain the gasoline or diesel fuel with a small amount of Di-Tertiary Butyl Peroxide (DTBP), 2-ethyl hexyl nitrate, or another cetane improver. An arrangement of this nature is useful since many reactivity modifiers are only needed in very dilute amounts, and thus a smaller tank for containing a reactivity modifier could be provided in a vehicle along with a conventional fuel tank arrangement, and with a metering arrangement that mixes a desired amount of reactivity modifier into the fuel line (or into a high-reactivity fuel line separate from a low-reactivity fuel line) when appropriate. To illustrate, a conventional diesel vehicle with a supplementary 1-2 quart tank containing DTBP would only require refilling every 3000-6000 miles or so, which is roughly the recommended frequency for an oil change, and thus the reactivity modifier tank could be refilled when the vehicle's oil is changed.
Use of the foregoing RCCI methodology tends to result in much lower peak combustion temperatures—as much as 40% lower—than in conventional diesel engines, owing to the increased control over the combustion process. This deters NOx formation, and additionally increases engine efficiency because less energy loss occurs from the engine through heat transfer. Further, the reactivities, amounts, and timing of the fuel charges can be adapted to optimize combustion such that there is less unburned fuel left at the end of the expansion stroke (and thus lost to the exhaust), thereby also enhancing engine efficiency, and also generating less soot.
Experimental results of the RCCI methods operating with diesel and gasoline fuels yielded a net indicated thermal efficiency of up to 53%, and a gross thermal efficiency of about 56%. (Thermal efficiency is a useful measure of fuel efficiency, as it represents the amount of fuel converted to output power by the engine, as opposed to being lost via heat transfer, exhaust, or other variables. Net thermal efficiency takes account of work output over the entire engine cycle, whereas gross thermal efficiency only takes account of the expansion and compression strokes, with an approximately 3% difference between the two being common.) In contrast, at the time the RCCI methods were first developed, the average conventional diesel engine had a thermal efficiency of approximately 42%, and the average gasoline engine had a thermal efficiency of approximately 25-30%. The RCCI methods therefore yielded exceptionally high fuel efficiency. At the same time, they met U.S. governmental soot emissions limits, NOx emissions limits, and fuel consumption limits for the year 2010 without the need for exhaust gas after-treatment. Emissions could be lowered even further with the implementation of measures such as exhaust after-treatment.
However, further experimentation found that with decreasing engine load, RCCI methods did not function as well. In versions of the invention using diesel fuel and gasoline, the invention required greater amounts of (higher-reactivity) diesel fuel and lesser amounts of (lower-reactivity) gasoline as load decreased. Below loads of approximately 4 bar indicated mean effective pressure (IMEP), and particularly at idle (i.e., less than about 1 bar), the engine effectively operated as a conventional diesel engine, with minimal or no use of gasoline. This yielded conventional low-load diesel performance, i.e., lower thermal efficiency and undesirably high emissions. The aforementioned U.S. Pat. No. 8,851,045 to Reitz et al. describes approaches for addressing this problem.
Further experimentation has similarly found that in some engines, particularly those with higher compression ratios, increasing engine load requires greater amounts of (lower-reactivity) gasoline and lesser amounts of (higher-reactivity) diesel fuel. This is problematic because one of the primary engine control mechanisms in RCCI is adjustment of the fuel ratio; thus, once the engine shifts solely to gasoline operation at higher load, control options are limited. The gasoline tends to prematurely autoignite, causing undesirably high levels of engine noise (knocking/ringing) and difficulty in control of combustion phasing (combustion start, rate, and duration). Attempts to achieve greater power output by injecting greater amounts of gasoline only exacerbates this problem. Greater control over combustion phasing can be attained by using measures such as greater amounts of EGR and lower intake air temperatures, but these measures can cause condensation of water vapor, hydrocarbons, and sulfur species along the air intake path, leading to fouling and corrosion of components along the path. It is therefore desirable to develop new combustion techniques which have many or all of the advantages of RCCI, while at the same time allowing operation at still higher loads.